So maybe there is a kernel of truth the rule, at least for men. Le's research focuses on commitment, including the factors associated with commitment and its role in promoting maintenance.

Also, science only looked at fertility in women up until recently.

Now science has studied mens ability to father children and discovered that there is an increased risk of miscarriage and birth deformities when the male partner is over 35.

Men, in contrast, are hypothesized to be most attracted to women in their reproductive prime, which tends to be when they are younger. Women’s preferences, on the other hand, hold relatively constant across their lives, not going more than a few years below their own age (extra-credit if you can identify the "cougar zone" in this figure), but women remain keen on men up to 10 years older than themselves. However, younger men (i.e., in their 20’s) tend to be married to someone of a similar age, but as they get older their wives get younger.